This invention relates to color picture tubes, and more particularly, to a formation method for a high-definition phosphor screen, and a photoresist composition with improved sensitivity and resolution for low illuminance exposure used in the method.
Normally, the phosphor screen of a color picture tube has phosphor layers of dots or stripes which emit red, green and blue light, and light absorbing layers which are provided between these phosphor layers.
Generally the pattern of these light absorbing layers is formed by a method such as the following. First, a photosensitive resin liquid is thinly coated on the inner surface of the faceplate of the picture tube. This coating is then dried into a resin film. Next, the resin film is exposed by the light from a dotted or linear light source through a shadow mask with a pluralitY of round or rectangular apertures. The exposed portions of the resin film are light-hardened into an insoluble photoresist layer, while the unexposed portions remain unhardened by the light. In order to generate light loci on the resin film approximating positions corresponding to the electron beam loci from an electron gun of a color picture tube, the light source is set in a position corresponding to the deflection centre of the electron gun. Moreover, a correction lens, which is normally composed of glass or transparent plastic, is provided between the light source and the shadow mask.
After this, the resin film is developed by a warm water spray. The unexposed resin film is removed by the spray and the corresponding portions of the inner surface of the faceplate are revealed.
In this condition, a slurry containing a light absorbing black material, such as graphite, next is coated on the inner surface of the faceplate. After this, the slurry is dried and forms a light absorbing film. Then, this film is placed in contact with a peeling agent. At this time, the light-hardened resin swells and becomes brittle due to this peeling agent. After this, when another warm water spray treatment is carried out, the brittle resin film and the light absorbing film on top of it are removed and the inner surface of the faceplate in those portions is revealed, while the other portions of the light absorbing film remain as they are. Thus, a dotted or striped black matrix is formed.
In recent years, the uses of color picture tubes have rapidly expanded to include OA equipment terminals, such as microcomputers, in addition to conventional use in televisions. In these applications, compared with the past, a high-definition phosphor screen is required. The actual dimensions vary depending on the size of the color picture tube. For instance, in the case of a 14-inch screen, whereas the hall pitch was (distance between adjacent dots) 0.6 mm in the past, it is now 0.2 mm in the case of a high-definition screen, and the dimensions are three times more highly defined. In order to accomplish this requirement, a photoresist composition which contains a sodium salt of 4.4'-bisazidestilbene-2.2'-disulphonic acid, polyvinylpyrrolidone and N- .beta.(aminoethyl).gamma.-aminopropyltrimethoxysilane as an adhesion accelerator(coupling agent), such as is disclosed in U.S. Pat. No. 3,917,794, has come into use.
On the other hand, the illuminance on the photoresist surface is greatly reduced during manufacture of this type of high-definition phosphor screen, because the photoresist is exposed by a light from a light source with a window of small diameter through a smaller aperture shadow mask to correspond to the higher definition. In the case of the above-mentioned 14-inch screen color picture tube, if the same 1 KW light source is used, the illuminance at the photoresist surface will be reduced to less than 10% of that in a conventional case. For example, exposure illuminance at the surface of the photoresist film for a conventional phosphor screen with a hole pitch of 0.6 mm is 1.7 mw/cm.sup.2, but exposure illuminance at the surface of the photoresist film for a high-definition phosphor screen with a hole pitch of 0.2 mm is 0.13 mw/cm.sup.2. Thus, if the exposure time was increased by 10 times in comparison to conventional processing in order to obtain the same amount of exposure, one would expect that the same shape of pattern could be obtained. However, in fact, even if much exposure is carried out by increasing the exposure time, the thickness of the hardened photoresist will not increase. Accordingly, there has been a problem in that a desired size of dot can nor be obtained. Although the cause of this phenomenon has not been clarified, it is assumed that a reciprocity law failure phenomenon of the photosensitive resin and the close contact between the resin film and the inner surface of the faceplate are intricately related.
Nevertheless, it is difficult accurately to form a high-definition black matrix pattern by the conventional method. Also, the sensitivity and resolution of these photosensitive resins are not sufficient to form a high definition screen.